Field of the Invention (Technical Field)
The present invention is in the field of an improved method for recovery of phosphorous, in particular of phosphorous from a waste stream, and to a product obtained thereby. The product is in a form wherein phosphorous can be released to, e.g., the soil and plants at a desired amount per interval of time.
Background of the Invention
Phosphorous is an element (P) that is used abundantly especially for growing crops, such as by adding fertilizer. To stress the significance of phosphorous it is noted that P fertilizer is essential for modern food production and is the limiting factor in crop yields. P is a critical global resource, as are water and energy resources. Phosphorus is considered essential for all living matter, including bacteria, plants and animals.
Phosphate rock, used as a source for P in fertilizers, clearly is a non-renewable resource that are likely to be depleted in 50-100 years, so there is a need to reuse phosphorus. Production peak is expected in about twenty years' time. Unlike other natural resources phosphorus has no substitute in food production. In view of a growing food demand such is even more of a concern. It is also noted that mining (of phosphate) is an energy intensive and polluting activity. Such will become even worse as the quality of phosphate rock is declining ([P2O5] in mined rock is decreasing and the concentration of associated heavy metals is increasing). It is noted that in principle heave metals need to removed, if applied as fertilizer, which is at the least energy intensive. One may conclude based on the above that cheap fertilizer is an element of the past. Alternative strategies need to be developed, as is the case.
For instance it is noted that human and animal excreta (urine and feces) are renewable and readily available sources of phosphorus. On top of that urine is essentially sterile and contains plant-available nutrients (P,N,K) in a correct ratio. Treatment and reuse is in principle simple but in practice still somewhat cumbersome, energy intensive and therefore expensive. For instance, removing high amounts of phosphorus at an end of a flow, such as at a wastewater treatment plant is expensive and energy intensive. It would be better if phosphorous would be captured at the source, e.g., a toilet, also in view of heavy metals.
In an extensive review paper Sartorius et al., in “Phosphorus Recovery from Wastewater—State-of-the-Art and Future Potential”, Int. Conference Nutrient recovery and management 2011, Inside and outside the fence, Jan. 9-12, 2011, Miami, Fla., USA, describe various routes for recovery of phosphorus. The mention that “very different approaches to the recovery of phosphorus from wastewater, sludge and ashes exist. These approaches differ by the origin of the used matter (wastewater, sludge liquor, fermented or non-fermented sludge ash) and the process precipitation, wet chemical extraction, and thermal treatment). They are characterized by their process steps, use of chemicals, complexity and effectiveness of the technology, economics, product quality for further use (fertilizer or industrial use), residuals, maturity of the technology, and degree of centralization and are rated positive, negative or neutral. Together these characteristics form the advantages and disadvantages of all the recovery processes”. So far none of these routes seem to be implemented fully.
US2013/0299420 A1 recites a method for recovering phosphate from sewage treatment plants using multi-stage anaerobic digestion includes the treatment of organic acid digest with calcium hydroxide, calcium oxide, and similar compounds to raise pH to near neutral values and precipitate calcium phosphate compounds such as brushite and similar amorphous compounds. The method includes the formation of calcium phosphates on weak-acid ion exchange columns and membranes in contact with organic acid digest. The system includes removal of the calcium phosphate compounds formed by sedimentation, either static or against an upwelling flow, centrifugation, or filtration. Under ideal conditions and surplus of calcium 50-90% (15/17 or ˜88%) of the phosphate may be captured, i.e., 10-40% remains in the sewage; these conditions are typically not reached, so incomplete capturing of phosphate occurs.
Various documents relate to capturing phosphorous. The two following documents relate to the so-called P-RoC process (Phosphorus Recovery by Crystallization from waste and process water) (as developed by the Karlsruher Institut für Technologie). According to the authors this process is capable of recovering about 50-60% of the P being present (Ehbrecht et al., 2nd European Sustainable Phosphorus Conference (ESPC2) Berlin, 5-6 Mar. 2015).
For instance EP 2 511 243 A1 recites a plant for phosphate removal from wastewater in a continuous operation, comprising a substantially cylindrical stirred reactor, which is charged with calcium silicate hydrate (CSH) as a crystallization substrate and a sedimentation container, where the stirred reactor is divided into a resting zone, which is located in the upper part and a reaction zone which is located in the lower part. In a reaction with phosphate comprising waste water three phosphate minerals are claimed to be formed, namely hydroxyl apatite, struvite (NH4MgPO4.6H2O), and brushite. The “P-elimination” in FIG. 2 shows a % between about 35% and 90% (or 65%-10% loss), for some reason depending on a reactor volume; such seems to reflect that the process is not scalable. FIG. 3 provides even worse results as a function of reaction time.
Ute Berg et al. in “Calcium silicate hydrate triggered phosphorous recovery—an efficient way to tap the potential of waste- and process waters as a key resource”, Internet citation, Jan. 1, 2006, pages 1747-1765 describes the use of a CSH for forming amongst others brushite. P-elimination of 30% to about 95% are reached in a laboratory fixed bed experiment, hence under ideal conditions. A fixed bed reactor is considered unsuited for a continuous process; even further after about 50 bed volumes the P-elimination drops dramatically. When waste water is used a maximum of about 90% is reached and a similar drop is observed. Mainly hydroxyl apatite was formed and some brushite.
The CSH materials are the main product of the hydration of Portland cement and are characterized by a typically complex structure and ratio between components, wherein a relative amount of Ca and Si are given and the oxygens are attributed to Ca and Si (or complexes thereof). Therein no individual Cao, SiO2 or hydrate can be distinguished; Ca, Si, H and O are part of a large complex, comparable to a crystal.
CSH materials may be used for pozzolanic reactions. The pozzolanic reaction is a chemical reaction that occurs in portland cement containing pozzolans. The pozzolanic reaction relates to a simple acid-base reaction between calcium hydroxide, also known as Portlandite, or (Ca(OH)2), and silicic acid (H4SiO4, or Si(OH)4); it is not performed in an aqueous environment. This has nothing to do with the present invention, but relates to a different field of technology, namely cement formation.
There is a need for improved methods for capturing phosphate from aqueous solutions; however, the prior art methods suffer from various drawbacks, such as sensitivity to impurities, such as heavy metals, organic material, etc., relatively high amounts of non-captured phosphorous, relative high costs per unit phosphorous captured, such as due to relative high amounts of reactants needed to capture relative low amounts of phosphorous, and energy, at the best a process that is optimized for a given phosphorous concentration range, capturing phosphorous in a form (typically and mostly struvite) that cannot be distributed easily through the soil, and non-robust processes.
Hence there still is a need for relative simple and effective process for recovering phosphorous, which reduces losses, and which overcomes one or more of the above mentioned disadvantages without jeopardizing beneficial characteristics.